Vibrio cholerae HlyA hemolysin is processed by proteolysis

Glu289 residue in the pore-forming motif of Vibrio cholerae cytolysin is important for efficient β-barrel pore formation

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging β-barrel pore-forming toxin (β-PFT). Upon binding to the target membranes, VCC monomers first assemble into oligomeric pre-pore intermediates, and subsequently transform into transmembrane β-barrel pores. VCC harbors a designated pore-forming motif, which, during oligomeric pore formation, inserts into the membrane and generates a transmembrane β-barrel scaffold. It remains an enigma how the molecular architecture of the pore-forming motif regulates the VCC pore-formation mechanism. Here, we show that a specific pore-forming motif residue, E289, plays crucial regulatory roles in the pore-formation mechanism of VCC. We find that the mutation of E289A drastically compromises pore-forming activity, without affecting the structural integrity and membrane-binding potential of the toxin monomers. Although our single-particle cryo-EM analysis reveals wild type-like oligomeric β-barrel pore formation by E289A-VCC in the membrane, we demonstrate that the mutant shows severely delayed kinetics in terms of pore-forming ability that can be rescued with elevated temperature conditions. We find that the pore-formation efficacy of E289A-VCC appears to be more profoundly dependent on temperature as compared to that of the wild type toxin. Our results suggest that the E289A mutation traps membrane-bound toxin molecules in the pre-pore-like intermediate state that is hindered from converting into the functional β-barrel pores by a large energy barrier, thus highlighting the importance of this residue for the pore-formation mechanism of VCC.

Characterization of V. cholerae O1 biotype El Tor serotype Ogawa possessing the ctxB gene of the classical biotype isolated from well water associated with the cholera outbreak in Kerala, South India

We investigated 22 water samples (17 well water and five pipe water - both chlorinated) and six soil samples from the surroundings of wells of the households of suspected patients from Palakkad district, Kerala (India), from where a cholera outbreak was reported during June-July 2016. A total of 25 Vibrio cholerae isolates were collected from three well water samples during a recent cholera outbreak. Biochemical and serological studies revealed that all of the isolates belonged to serogroup O1, biotype El Tor, serotype Ogawa. PCR assays confirmed the occurrence of ctxB, ctxA, hlyA, tcpA El Tor,VPI, ace, zot, ompW, rfbO1 and toxR genes in all isolates. The presence of the ctxB gene of the classical biotype in all of the El Tor isolates suggests that it is a new variant of El Tor biotype. Antibiogram profile of all V. cholerae O1 isolates revealed resistance towards five classes of antibiotics island and indicates that they were multidrug resistant. ERIC-PCR and PFGE finger prints showed the clonal relationship among the V. cholerae O1 isolates. The results of this study revealed the emergence of a new variant of El Tor biotype in the water samples from Palakkad district, from where a cholera outbreak was reported.

Identification of the target DNA sequence and characterization of DNA binding features of HlyU, and suggestion of a redox switch for hlyA expression in the human pathogen Vibrio cholerae from in silico studies

HlyU, a transcriptional regulator common in many Vibrio species, activates the hemolysin gene hlyA in Vibrio cholerae, the rtxA1 operon in Vibrio vulnificus and the genes of plp-vah1 and rtxACHBDE gene clusters in Vibrio anguillarum. The protein is also proposed to be a potential global virulence regulator for V. cholerae and V. vulnificus. Mechanisms of gene control by HlyU in V. vulnificus and V. anguillarum are reported. However, detailed elucidation of the interaction of HlyU in V. cholerae with its target DNA at the molecular level is not available. Here we report a 17-bp imperfect palindrome sequence, 5′-TAATTCAGACTAAATTA-3′, 173 bp upstream of hlyA promoter, as the binding site of HlyU. This winged helix-turn-helix protein binds necessarily as a dimer with the recognition helices contacting the major grooves and the β-sheet wings, the minor grooves. Such interactions enhance hlyA promoter activity in vivo. Mutations affecting dimerization as well as those in the DNA–protein interface hamper DNA binding and transcription regulation. Molecular dynamic simulations show hydrogen bonding patterns involving residues at the mutation sites and confirmed their importance in DNA binding. On binding to HlyU, DNA deviates by ∼68º from linearity. Dynamics also suggest a possible redox control in HlyU.

Genomic and phenotypic characterization of Vibrio cholerae non-O1 isolates from a US Gulf Coast cholera outbreak

Between November 2010, and May 2011, eleven cases of cholera, unrelated to a concurrent outbreak on the island of Hispaniola, were recorded, and the causative agent, Vibrio cholerae serogroup O75, was traced to oysters harvested from Apalachicola Bay, Florida. From the 11 diagnosed cases, eight isolates of V. cholerae were isolated and their genomes were sequenced. Genomic analysis demonstrated the presence of a suite of mobile elements previously shown to be involved in the disease process of cholera (ctxAB, VPI-1 and -2, and a VSP-II like variant) and a phylogenomic analysis showed the isolates to be sister taxa to toxigenic V. cholerae V51 serogroup O141, a clinical strain isolated 23 years earlier. Toxigenic V. cholerae O75 has been repeatedly isolated from clinical cases in the southeastern United States and toxigenic V. cholerae O141 isolates have been isolated globally from clinical cases over several decades. Comparative genomics, phenotypic analyses, and a Caenorhabditis elegans model of infection for the isolates were conducted. This analysis coupled with isolation data of V. cholerae O75 and O141 suggests these strains may represent an underappreciated clade of cholera-causing strains responsible for significant disease burden globally.

Ecology and genetic structure of a northern temperate Vibrio cholerae population related to toxigenic isolates

Although Vibrio cholerae is an important human pathogen, little is known about its populations in regions where the organism is endemic but where cholera disease is rare. A total of 31 independent isolates confirmed as V. cholerae were collected from water, sediment, and oysters in 2008 and 2009 from the Great Bay Estuary (GBE) in New Hampshire, a location where the organism has never been detected. Environmental analyses suggested that abundance correlates most strongly with rainfall events, as determined from data averaged over several days prior to collection. Phenotyping, genotyping, and multilocus sequence analysis (MLSA) revealed a highly diverse endemic population, with clones recurring in both years. Certain isolates were closely related to toxigenic O1 strains, yet no virulence genes were detected. Multiple statistical tests revealed evidence of recombination among strains that contributed to allelic diversity equally as mutation. This relatively isolated population discovered on the northern limit of detection for V. cholerae can serve as a model of natural population dynamics that augments predictive models for disease emergence.

The Vibrio cholerae cytolysin promotes chloride secretion from intact human intestinal mucosa

Background

The pathogenicity of the Vibrio cholerae strains belonging to serogroup O1 and O139 is due to the production of virulence factors such as cholera toxin (CT) and the toxin-coregulated pilus (TCP). The remaining serogroups, which mostly lack CT and TCP, are more frequently isolated from aquatic environmental sources than from clinical samples; nevertheless, these strains have been reported to cause human disease, such as sporadic outbreaks of watery diarrhoea and inflammatory enterocolitis. This evidence suggested the possibility that other virulence factor(s) than cholera toxin might be crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea, but their nature remains unknown. VCC, the hemolysin produced by virtually all Vibrio cholerae strains, has been proposed as a possible candidate, though a clear-cut demonstration attesting VCC as crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea is still lacking.

Methodology/Principal Findings

Electrophysiological parameters and paracellular permeability of stripped human healthy colon tissues, obtained at subtotal colectomy, mounted in Ussing chamber were studied in the presence or absence of VCC purified from culture supernatants of V. cholerae O1 El Tor strain. Short circuit current (I_SC) and transepithelial resistance (R_T) were measured by a computerized voltage clamp system. The exposure of sigmoid colon specimens to 1 nM VCC resulted in an increase of I_SC by 20.7%, with respect to the basal values, while R_T was reduced by 12.3%. Moreover, increase in I_SC was abolished by bilateral Cl⁻ reduction.

Conclusion/Significance

Our results demonstrate that VCC, by forming anion channels on the apical membrane of enterocytes, triggers an outward transcellular flux of chloride. Such an ion movement, associated with the outward movement of Na⁺ and water, might be responsible for the diarrhoea caused by the non-toxigenic strains of Vibrio cholerae.

The Vibrio cholerae cytolysin promotes activation of mast cell (T helper 2) cytokine production

Many strains of Vibrio cholerae produce a cytolysin (VCC) that forms oligomeric transmembrane pores responsible for vacuolization of several cell types in culture. Here we suggest that VCC could contribute to the T helper 2 (Th2) response seen in the natural infection; acting through TLR2, VCC enhances mast cells secretion of IL-4, IL-6 and TNF-alpha by 330-, 290- and 550-fold respectively. Moreover, VCC-induced cytokine production is dependent on increased cytosolic Ca(2+) and on the presence of the Src family kinases Lyn and Fyn, known to be required for FcepsilonRI-dependent activation of mast cells. These findings strongly suggest that VCC has a pro-inflammatory activity promoting a Th2-type immune profile.

Pore formation by Vibrio cholerae cytolysin requires cholesterol in both monolayers of the target membrane

Vibrio cholerae cytolysin (VCC) forms oligomeric transmembrane pores in cholesterol-rich membranes. To better understand this process, we used planar bilayer membranes. In symmetric membranes, the rate of the channel formation by VCC has a superlinear dependency on the cholesterol membrane fraction. Thus, more than one cholesterol molecule can facilitate VCC-pore formation. In asymmetric membranes, the rate of pore formation is limited by the leaflet with the lower cholesterol content. Methyl-beta-cyclodextrin, which removes cholesterol from membranes, rapidly inhibits VCC pore formation, even when it is added to the side opposite that of VCC addition. The results suggest that cholesterol in both membrane leaflets aid VCC-pore formation and that either leaflet can function as a kinetic bottleneck with respect to the rate of pore-formation.

Haemolysin produced by Vibrio mimicus activates two Cl- secretory pathways in cultured intestinal-like Caco-2 cells

Haemolysin (VMH) is a virulent factor produced by Vibrio mimicus, a human pathogen that causes diarrhoea. As intestinal epithelial cells are the primary targets of haemolysin, we investigated its effects on ion transport in human colonic epithelial Caco-2 cells. VMH increased the cellular short circuit current (Isc), used to estimated ion fluxes, and 125I efflux of the cells. The VMH-induced increases in Isc and 125I efflux were suppressed by depleting Ca2+ from the medium or by pretreating the cells with BAPTA-AM or by Rp-adenosin 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS). The Cl- channel inhibitors 4,4'-disothiocyanatostibene-2,2'-disulfonic acid (DIDS), glybenclamide, and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) suppressed the VMH-induced increases in Isc and 125I efflux. Moreover, VMH increased the intracellular concentrations of Ca2+ and cAMP. Thus, VMH stimulates Caco-2 cells to secrete Cl- by activating both Ca2+ -dependent and cAMP-dependent Cl- secretion mechanisms. VMH forms ion-permeable pores in the lipid bilayer that are non-selectively permeable to small ions. However, the ion permeability of these pores was not inhibited by glybenclamide and DIDS, and VMH did not change the cell membrane potential. These observations indicate that the pores formed on the cell membrane by VMH are unlikely to be involved in VMH-induced Cl- secretion. Notably, VMH stimulated fluid accumulation in the iliac loop test that was fully suppressed by a combination of DIDS and glybenclamide. Thus, Ca2+-dependent and cAMP-dependent Cl- secretion may be important therapeutic targets with regard to the diarrhoea that is induced by Vibrio mimicus.

Crystal Structure of the Vibrio cholerae Cytolysin (VCC) Pro-toxin and its Assembly into a Heptameric Transmembrane Pore

Pathogenic Vibrio cholerae secrete V. cholerae cytolysin (VCC), an 80 kDa pro-toxin that assembles into an oligomeric pore on target cell membranes following proteolytic cleavage and interaction with cell surface receptors. To gain insight into the activation and targeting activities of VCC, we solved the crystal structure of the pro-toxin at 2.3A by X-ray diffraction. The core cytolytic domain of VCC shares a fold similar to the staphylococcal pore-forming toxins, but in VCC an amino-terminal pro-domain and two carboxy-terminal lectin domains decorate the cytolytic domain. The pro-domain masks a protomer surface that likely participates in inter-protomer interactions in the cytolytic oligomer, thereby explaining why proteolytic cleavage and movement of the pro-domain is necessary for toxin activation. A single beta-octyl glucoside molecule outlines a possible receptor binding site on one lectin domain, and removal of this domain leads to a tenfold decrease in lytic activity toward rabbit erythrocytes. VCC activated by proteolytic cleavage assembles into an oligomeric species upon addition of soybean asolectin/cholesterol liposomes and this oligomer was purified in detergent micelles. Analytical ultracentrifugation and crystallographic analysis indicate that the resulting VCC oligomer is a heptamer. Taken together, these studies define the architecture of a pore forming toxin and associated lectin domains, confirm the stoichiometry of the assembled oligomer as heptameric, and suggest a common mechanism of assembly for staphylococcal and Vibrio cytolytic toxins.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

The Vibrio cholerae haemolysin anion channel is required for cell vacuolation and death

Several strains of Vibrio cholerae secrete a haemolytic toxin of 63 kDa, termed V. cholerae cytolysin (VCC). This toxin causes extensive vacuolation and death of cells in culture and forms an anion-selective channel in planar lipid bilayers and in cells. Here, we identify inhibitors of the VCC anion channel and show that the formation of the anion channel is necessary for the development of the vacuoles and for the cell death induced by this toxin. Using markers of cell organelles, we show that vacuoles derive from different intracellular compartments and we identify the contribution of late endosomes and of the trans-Golgi network in vacuole biogenesis.

Trialysin, a novel pore-forming protein from saliva of hematophagous insects activated by limited proteolysis

We have characterized a pore-forming lytic protein from the saliva of the hematophagous insect Triatoma infestans, a vector of Chagas disease. This protein, named trialysin, has 22 kDa and is present in the saliva at about 200 microg/ml. Purified trialysin forms voltage-dependent channels in planar lipid bilayers with conductance of 880 +/- 40 pS. It lyses protozoan parasites and bacteria indicating that it has a role in the control of microorganism growth in the salivary glands. At higher concentrations, but below those found in saliva, trialysin can also permeabilize and lyse mammalian cells, suggesting that it might also facilitate insect blood feeding by interfering with the cell response of the host. The translated cDNA sequence of trialysin shows a basic, lysine-rich protein in which the N-terminal region is predicted to form an amphipathic alpha-helical structure with positive charges on one side and hydrophobic amino acids on the opposite side. A synthetic peptide corresponding to this cationic amphipathic alpha-helix induces protozoan lysis and mammalian cell permeabilization, showing that this region is involved in lytic activity. However, the lytic peptide G6V32 is 10-fold less efficient than trialysin in lysing parasites and 100-fold less efficient in permeabilizing mammalian cells. Trialysin activity is about 10-fold reduced in salivary gland homogenates prepared in the presence of an irreversible serine-protease inhibitor. Since trialysin precursor contains an anionic pro-sequence of 33 amino acids contiguous to the cationic amphipathic putative alpha-helix, we propose that removal of the acidic pro-sequence by limited proteolysis activates trialysin by exposing this lytic basic amphipathic motif.

Oligomerization of Vibrio cholerae cytolysin yields a pentameric pore and has a dual specificity for cholesterol and sphingolipids in the target membrane

Vibrio cholerae cytolysin permeabilizes animal cell membranes. Upon binding to the target lipid bilayer, the protein assembles into homo-oligomeric pores of an as yet unknown stoichiometry. Pore formation has been observed with model liposomes consisting of phosphatidylcholine and cholesterol, but the latter were much less susceptible to the cytolysin than were erythrocytes or intestinal epithelial cells. We here show that liposome permeabilization is strongly promoted if cholesterol is combined with sphingolipids, whereby the most pronounced effects are observed with monohexosylceramides and free ceramide. These two lipid species are prevalent in mammalian intestinal brush border membranes. We therefore propose that, on its natural target membranes, the cytolysin has a dual specificity for both cholesterol and ceramides. To assess the stoichiometry of the pore, we generated hybrid oligomers of two naturally occurring variants of the toxin that differ in molecular weight. On SDS-polyacrylamide gel electrophoresis, the mixed oligomers formed a pattern of six distinct bands. Ordered by decreasing electrophoretic mobility, the six oligomer species must comprise 0 to 5 subunits of the larger form; the pore thus is a pentamer. Due to both lipid specificity and pore stoichiometry, V. cholerae cytolysin represents a novel prototype in the class of bacterial pore-forming toxins.

Characterization of the Vibrio cholerae El Tor lipase operon lipAB and a protease gene downstream of the hly region

We have cloned and sequenced a region encoding a lipase operon and a putative, previously uncharacterized metalloprotease of Vibrio cholerae O1. These lie downstream of hlyA and hlyB, which encode the El Tor hemolysin and methyl-accepting chemotactic factor, respectively. Previous reports identified the hlyC gene downstream of hlyAB, encoding an 18.3-kDa protein. However, we now show that this open reading frame (ORF) encodes a 33-kDa protein, and since the amino acid sequence is highly homologous to the triacylglyceride-specific lipase of Pseudomonas spp., hlyC has been renamed lipA. LipA contains the highly conserved pentapeptide and catalytic triad amino acid regions of the catalytic sites of other lipases. The region downstream of lipA has been sequenced and has revealed ORFs lipB and prtV. The amino acid sequence of lipB is homologous to those of the accessory lipase proteins (lipase-specific foldase) required by Pseudomonas and various other bacterial species for the production of mature active lipase, and in agreement with this, we show that both lipA and lipB are required to restore a lipase-deficient lipA null mutant of V. cholerae. The intergenic stop codon for lipA overlaps the ribosome-binding site for lipB, and a stem-loop resembling a rho-independent terminator is present immediately downstream from lipB, suggesting that lipA and lipB form a lipase operon in V. cholerae. prtV lies downstream of lipAB but is transcribed in the opposite direction and is predicted to share the same putative transcriptional terminator with lipAB. The zinc-binding and catalytic domains conserved among many metalloproteases are present in PrtV, which is highly homologous to the immune inhibitor A (InA) metalloprotease of Bacillus thuringiensis. PrtV was visualized as approximately 102 kDa, which is consistent with the coding capacity of the gene. The genetic organization of this region suggests that it is possibly part of a pathogenicity island, encoding products capable of damaging host cells and/or involved in nutrient acquisition by V. cholerae. However, neither lipA nor prtV null mutants were attenuated in the infant mouse model, nor did they exhibit reduced colonization potential compared with wild type in competition experiments.

Mode of primary binding to target membranes and pore formation induced by Vibrio cholerae cytolysin (hemolysin)

Vibrio cholerae cytolysin (VCC) is produced by many non-choleratoxigenic strains of V. cholerae, and possibly represents a relevant pathogenicity determinant of these bacteria. The protein is secreted as a pro-toxin that is proteolytically cleaved to yield the active toxin with a molecular mass of approximately 63 kDa. We here describe a simple procedure for preparative isolation of mature VCC from bacterial culture supernatants, and present information on its mode of binding and pore formation in biological membranes. At low concentrations, toxin monomers interact with a high-affinity binding site on highly susceptible rabbit erythrocytes. This as yet unidentified binding site is absent on human erythrocytes, which are less susceptible to the toxin action. At higher concentrations, binding of the toxin occurs to both rabbit and human erythrocytes in a non-saturable manner. Cell-bound toxin monomers oligomerize to form supramolecular structures that are seen in the electron microscope as apparently hollow funnels, and oligomerization correlates functionally with the appearance of small transmembrane pores. Osmotic protection experiments indicate that the toxin channels are of finite size with a diameter of 1-2 nm. The mode of action of VCC closely resembles that of classical pore-forming toxins such as staphylococcal alpha-toxin and the aerolysin of Aeromonas hydrophila.

Purification and characterization of a hemolysin produced by Vibrio mimicus

Vibrio mimicus is a causative agent of human gastroenteritis. This pathogen secretes a pore-forming toxin, V. mimicus hemolysin (VMH), which causes hemolysis by three sequential steps: binding to an erythrocyte membrane, formation of a transmembrane pore, and disruption of the cell membrane. VMH with a molecular mass of 63 kDa was purified by ammonium sulfate precipitation and column chromatography with phenyl Sepharose HP and Superose 6 HR. The hemolytic reaction induced by VMH continued up to disruption of all erythrocytes in the assay system. Moreover, VMH that bound preliminarily to erythrocyte ghosts showed a sufficient ability to attack intact erythrocytes. These results suggest reversible binding of the toxin molecule to the membrane. The final cell-disrupting stage was effectively inhibited by various divalent cations. Additionally, some cations, such as Zn2+ and Cu2+, blocked the pore-forming stage at high concentrations. Although VMH could disrupt all kinds of mammalian erythrocytes tested, those from horses were most sensitive to the hemolysin. Horse erythrocytes were found to have the most toxin-binding sites and to be hemolyzed by the least amount of membrane-bound toxin molecules, suggesting that toxin binding to and pore formation on erythrocytes are more effective in horses than in other mammals. Purified VMH induced fluid accumulation in a ligated rabbit ileal loop in a dose-dependent manner. In addition, the antibody against the hemolysin obviously reduced enteropathogenicity of living V. mimicus cells. These findings clearly demonstrate that VMH is probably involved in the virulence of this human pathogen.

Potent membrane-permeabilizing and cytocidal action of Vibrio cholerae cytolysin on human intestinal cells

Many strains of Vibrio cholerae non-O1 and O1 El Tor that cause diarrhea do not harbor genes for a known secretogenic toxin. However, these strains usually elaborate a pore-forming toxin, hitherto characterized as a hemolysin and here designated V. cholerae cytolysin, whose action on intestinal cells has not yet been described. We report that V. cholerae cytolysin binds as a monomer to Intestine 407 cells and then assembles into detergent-stable oligomers that probably represent tetra- or pentamers. Oligomer formation is accompanied by generation of small transmembrane pores that allow rapid flux of K+ but not influx of Ca2+ or propidium iodide. Pore formation is followed by irreversible ATP depletion and cell death. Binding of fewer than 10(4) toxin molecules per cell in vitro is lethal. The possibility is raised that production of this toxin by bacteria that are in close contact with intestinal cells is rapidly cytocidal in vivo, and death of intestinal cells may be a cause of diarrhea.

Biochemical and physiological characteristics of HlyA, a pore-forming cytolysin of Vibrio cholerae serogroup O1

Among the various toxins produced by the bacterial species Vibrio cholerae is HlyA, a cytolytic protein commonly called the E1 Tor hemolysin. HlyA is synthesized and processed in a complex manner involving various processed or degraded forms, that may co-purify and complicate the interpretation of biochemical and physiological experiments. In this study a single form of HlyA was purified by gel filtration and chromatofocusing using fast protein liquid chromatography in the presence of protease inhibitors. A 45-fold purification was obtained, with a final recovery of 17% of pure 60,000 mol. wt HlyA. A significant improvement in specific activity to 8.5 x 10(6) Chinese hamster ovary tissue culture units per mg protein was obtained. Physiological activity studies indicated that cytolysis of erythrocytes (hemolysis) was inhibited by oxygen: storage of HlyA under oil, and experimentation in N2-flushed buffers maintained activity. HlyA-mediated lysis of human erythrocytes was characterized by a significant lag phase, followed by a rapid induction of hemolysis. Hemolysis was inhibited by sucrose, an osmotic protectant, suggesting that the initial action of HlyA on erythrocytes is to raise the basal cation permeability of the cell membrane. The most likely cytolytic mechanism is thus the formation of transmembrane lesions such as homopolymer pores in target cells, as has been found for toxins from numerous other bacterial pathogens.

Membrane attack induced by HlyA, a pore-forming toxin of Vibrio cholerae

Determining the activity of purified toxins has generally provided the basis for establishing their role in the host-pathogen relationship. The bacterial genus Vibrio produces a number of exotoxins in addition to cholera toxin, including haemolysin A (HlyA; Vibrio cholerae) and thermostable direct haemolysin (TDH; Vibrio parahaemolyticus), both of which possess membrane-targeting cytolytic activity. The action of HlyA has been analyzed using protocols previously applied to TDH: lysis and flux experiments on human erythrocytes showed that HlyA similarly causes lysis after cell swelling (by colloid osmosis) due to an elevation of cation permeability. However, kinetic measurements of flux, haemolysis and cation selectivity showed that HlyA and TDH form pores with distinct and characteristic features.

In vitro proteolytic processing and activation of the recombinant precursor of El Tor cytolysin/hemolysin (pro-HlyA) of Vibrio cholerae by soluble hemagglutinin/protease of V. cholerae, trypsin, and other proteases

Vibrio cholerae produces a cytolytic toxin named El Tor cytolysin/hemolysin which is encoded by the hlyA gene. This cytolysin is produced as a 79-kDa precursor form (pro-HlyA) into the culture supernatant after cleavage of the signal peptide of the hlyA product (prepro-HlyA). The pro-HlyA is then processed to a 65-kDa mature cytolysin (mature HlyA) after cleavage of the 15-kDa N-terminal peptide (pro region) of the 79-kDa precursor, usually at the bond between Ala-157 and Asn-158. We investigated whether proteases could process the recombinant 79-kDa pro-HlyA to the 65-kDa mature HlyA. We observed that the soluble hemagglutinin/ protease (HA/protease; a major protease of V. cholerae), trypsin, alpha-chymotrypsin, subtilisin BPN', papain, and thermolysin all processed the pro-HlyA to the 65-kDa mature form of the protein. Along with this, the protease-processed HlyA showed drastically increased hemolytic activity. The N-terminal amino acid of the mature form of cytolysin generated by HA/protease was Phe-151, and that due to trypsin was Ser-149. Other proteases also cleaved the pro-HlyA at a nearby site, between Leu-146 and Ser-153, and all the processed cytolysins showed increased hemolytic activity. These data suggest that the active El Tor cytolysin of V. cholerae could be derived from the C-terminal region of a pro-HlyA following proteolytic cleavage of the bonds in the vicinity of Leu-146 to Asn-158 by any of a wide variety of proteases.

HlyA hemolysin of Vibrio cholerae O1 biotype E1 Tor. Identification of the hemolytic complex and evidence for the formation of anion-selective ion-permeable channels

Hemolysin (HlyA) was concentrated from supernatants of different Vibrio cholerae O1 biotype E1 Tor strains by ammonium sulfate precipitation. The concentration of the toxin in the supernatants and in the precipitates was quantified using its hemolytic activity. The toxin formed a high molecular-mass band (about 220 kDa) on SDS/PAGE while the toxin monomer had a molecular mass of 60 kDa when it was heated. The addition of the E1 Tor hemolysin oligomers, but not that of the monomers, to the aqueous phase bathing lipid bilayer membranes resulted in the formation of ion-permeable channels, which had long lifetimes at small voltages. The hemolysin channel had a single-channel conductance of 350 pS in 1 M KCl. These results defined hemolysin (HlyA) from V. cholerae as a channel-forming component with properties similar to other cytolytic toxins. The long lifetime of the channel suggested that the channel-forming oligomer did not show a rapid association/dissociation reaction. At voltages larger than 50 mV, the hemolysin channel was voltage dependent in an asymmetric fashion dependent on the side of its addition. The single-channel conductance of the hemolysin (HlyA) from V. cholerae O1 biotype E1 Tor channel was a linear function of the bulk aqueous conductance, which suggested that the toxin forms aqueous channels with an estimated minimum diameter of about 0.7 nm. The hemolysin channel of V. cholerae was found to be moderately anion-selective. The pore-forming properties of hemolysin (HlyA) from V. cholerae O1 biotype E1 Tor were compared with those of aerolysin of Aeromonas sobria and alpha-toxin from Staphylococcus aureus. All these cytolytic toxins must probably oligomerize for activity in biological and artificial membranes and form anion-selective channels.

Aberrant gene for E1 Tor hemolysin from Vibrio cholerae non-O1, N037

Vibrio cholerae non-O1 strain N037 produced a hemolysin (NO37-Hly) which was antigenically similar to E1 Tor hemolysin (E1 Tor-Hly) but different in molecular size, hemolytic activity, and glucose binding capacity. In the gene encoding NO37-Hly, a 4-bp insertion into the structural gene for E1 Tor-Hly (hlyA) was found. The insertion in a shift of codon frames generating a new stop codon in the downstream region. NO37-Hly was a truncated product of E1 Tor-Hly sharing 90% of the N terminal region. This suggested that the 10% C-terminal region of E1 Tor-Hly is needed for the maximal hemolytic activity and glucose binding capacity.

Characterization of Vibrio cholerae El Tor cytolysin as an oligomerizing pore-forming toxin

V. cholerae El Tor cytolysin is a secreted, water-soluble protein of M(r) 60,000 that may be relevant to the pathogenesis of acute diarrhea. In this communication, we demonstrate that the toxin binds to and oligomerizes in target membranes to form SDS-stable aggregates of M(r) 200,000-250,000 that generate small transmembrane pores. Pores formed in erythrocytes were approximately 0.7 nm in size, as demonstrated by osmotic protection experiments. Binding was shown to occur in a temperature-independent manner preceding the temperature-dependent oligomerization step. Pores were also shown to be formed in L929 and HEp-2 cells, human fibroblasts and keratinocytes, albeit with highly varying efficacy. At neutral pH and in the presence of serum, human fibroblasts were able to repair a limited number of lesions. The collective data identify V. cholerae El Tor cytolysin as an oligomerizing toxin that damages cells by creating small transmembrane pores.

The transcriptional activator HlyU of Vibrio cholerae: nucleotide sequence and role in virulence gene expression

HlyU upregulates expression of the haemolysin, HlyA, of Vibrio cholerae. DNA sequence analysis indicates that HlyU is an 11.9 kDa protein containing a putative helix-turn-helix motif and belonging to a family of small regulatory proteins, including NoIR (Rhizobium meliloti), SmtB (Synechococcus PCC 7942) and ArsR (plasmids R773, Escherichia coli; pI258, Staphylococcus aureus; and pSX267, Staphylococcus xylosus). An hlyU mutant was constructed by insertional inactivation, and found to be deficient in the production of both the haemolysin and a 28 kDa secreted protein. The mutant was assessed for virulence in the infant mouse cholera model, revealing a 100-fold increase in the LD50. This suggests that HlyU promotes expression of virulence determinant(s) in vivo.

Aggregative adherence fimbria I expression in enteroaggregative Escherichia coli requires two unlinked plasmid regions

Adherence to HEp-2 cells by many enteroaggregative Escherichia coli (EAggEC) strains is associated with the expression of flexible, bundle-forming fimbriae 2 to 3 nm in diameter, designated aggregative adherence fimbriae I (AAF/I). We have previously reported the molecular cloning and TnphoA mutagenesis of AAF/I genes from the large plasmid of prototype EAggEC strain 17-2 (J. P. Nataro, Y. Deng, D. R. Maneval, A. L. German, W. C. Martin, and M. M. Levine, Infect. Immun. 60:2297-2304, 1992). Here, we report that further mapping and subcloning of AAF/I regions suggest that expression of the fimbriae requires two separate plasmid regions (designated regions 1 and 2). Approximately 9 kb of DNA unnecessary for fimbrial expression separates the two regions; this intervening segment encodes the EAggEC heat-stable enterotoxin (EAST1). Neither region was capable of conferring aggregative HEp-2 adherence (AA) when cloned individually; when the two regions were cloned as a single fragment or when each was cloned into a different vector and introduced into the same E. coli HB101 cell, AA was restored. AA-positive constructs also expressed human erythrocyte hemagglutination, autoagglutination in broth cultures, and the production of AAF/I as detected by immunogold electron microscopy.

Incidence of toxigenic vibrios in foods available in Taiwan

A total of 1088 vibrios and related species were isolated from seafood and aquacultured foods available in Taiwan. They were identified as Vibrio alginolyticus, V. cholerae, V. fluvialis I, V. fluvialis II, V. parahaemolyticus, V. mimicus, Aeromonas caviae, A. hydrophila, A. sobria and other species. Incidence of these Vibrio and Aeromonas species in these foods was high. Vibrio parahaemolyticus was frequently found in seawater and in foods of freshwater origin. The Vibrio isolates were examined for enzymatic and toxigenic activities. Most of them showed strong lipase or protease activities. Haemolytic activities of V. cholerae, V. fluvialis I and V. fluvialis II isolates were mostly strong. About 49% showed cytotoxic activity and 5% cytotonic activity in Chinese hamster ovary cell culture assay. Nevertheless, only three non-O1 V. cholerae (2.07%) and two V. parahaemolyticus isolates (1.65%) produced cholera toxin and thermostable direct haemolysin activity, respectively. Various toxigenic vibrios may be important food-borne pathogens in this region because of their high incidence in foods.

Transcription of the Vibrio cholerae haemolysin gene, hlyA, and cloning of a positive regulatory locus, hlyU

Transcription of the Vibrio cholerae hlyA gene, which encodes a cytotoxic haemolysin, has been investigated. The hlyA transcript initiates 430 nucleotides (nt) upstream of the translational start site. hlyA-cat transcriptional fusion constructs were active in V. cholerae but not in Escherichia coli. An hlyA-cat fusion was used to select, from a V. cholerae O17 plasmid library, a clone that could activate the hlyA promoter in E. coli. This regulatory locus has been designated hlyU. hlyU appears to be distinct from the previously described hlyR locus.

Pore-forming bacterial protein hemolysins (cytolysins)

Protein toxins forming pores in biological membranes occur frequently in Gram-positive and Gram-negative bacteria. They kill either bacteria or eukaryotic cells (at most, a few seem to act on both groups of organisms). Most of the toxins affecting eukaryotes have clearly been shown to be related to the pathogenicity of the producing organisms. Toxin formation frequently involves a number of genes which encode the toxin polypeptide as well as proteins for toxin activation and secretion. Regulation of toxin production is usually coupled with that of the synthesis of a number of other virulence factors. Iron is the only known environmental factor that regulates transcription of a number of toxin genes by a Fur repressor-type mechanism, as has been originally described in Escherichia coli. Interestingly, the thiol-activated hemolysins (cytolysins) of Gram-positive bacteria contain a single cysteine which can be replaced by alanine without affecting the cytolytic activity. The Gram-negative hemolysins (cytolysins) are usually synthesized as precursor proteins, then covalently modified to yield an active hemolysin and secreted via specific export systems, which differ for various types of hemolysins.